Tire curing molds generally comprise an upper and lower half mold connectable in such a manner as to define a toroidal chamber for receiving a green tire for curing, inside which tire a bladder is inflated, usually in two successive stages, so that the outer surface of the tire adheres to the inner surface of the mold, and also for defining a toroidal passage for a high-temperature heat exchange fluid.
Due to the mechanical and thermal stresses to which they are subjected and the particular function they are called upon to perform, bladders of the aforementioned type demand an extremely high degree of precision and are mostly either handcrafted or injection molded.
A less expensive known method of producing such bladders is by what is known as "compression molding." According to this process, the bladders are produced using a curing mold comprising two half molds and, possibly, an inner core, which, when the mold is closed, define an inner toroidal chamber of exactly the same shape as the bladder being produced. Prior to closing the mold, a mass of uncured or green rubber is fed inside one of the half molds and, when the mold is closed, is subjected to relatively high pressure so as to force it to move and completely fill the inner chamber.
The above compression molding process involves a number of drawbacks, foremost of which are the need for relatively high-capacity presses for closing molds, and the overall inaccuracy of the finished product. The latter is due to variations in the shape and volume of the inner chamber during actual molding, which inaccuracy results in friction between the moving rubber and the walls of the inner chamber, thus preventing the rubber from faithfully reproducing the shape of the inner chamber when the mold is closed.